(1) Field of the Invention
The present invention is directed generally towards energy harvesting utilizing ferroelectric piezocrystal material transducers, and in particular, to a ferroelectric generator that utilizes a crystalline phase transformation to achieve higher energy density.
(2) Description of the Prior Art
While piezoelectric materials have been successfully utilized in sensors and actuators, their use as practical power sources for generating a useful amount of electricity in portable generators has been limited by the small amounts of power available and the efficiency of generating this power. Most practically applied piezoelectric energy harvesting is performed by piezoelectric composite materials. These materials incorporate a plurality of piezoelectric crystals in a matrix material. Efficiency is limited by the matrix material, coupling inefficiencies and crystal orientations.
Single crystal ferroelectric materials are also known in the art for energy harvesting. While more efficient than composite materials, these materials operate in the linear region of the ferroelectric response curve because the linear region is the operating region without pre-stress or bias electric field.
The current most promising class of materials for energy harvesting are relaxor-ferroelectric single crystals. These materials are single crystals of ferroelectric materials (for example, lead zinc niobate-lead titanate, known hereinafter as PZN-PT). These materials have been shown to deliver a high voltage at greater efficiency when the crystal is subjected to stress. In some special compositions (for example certain compositions of ternary lead indium niobate-lead magnesium niobate-lead titanate (PIN-PMN-PT)), the material will undergo a phase transformation accompanied by a very sharp hysteretic strain and a dramatic change in stiffness when subjected to external stress. This phase transformation can be invoked repeatedly at variable rates to induce large strains in the single crystal element. Known compositions exhibiting this type of phase change behavior include (1-x)PZN-xPT where 0.04<x<0.11. The composition where x was 0.06 has been tested at multiple temperatures and applied direct current (DC) bias fields. Under these conditions, it has been shown to exhibit a phase transformation.
It is known to combine single crystal piezoelectric materials with mechanical stress inducing means. This is typically performed in order to avoid putting the piezoelectric material in tensile stress because of the fragility of ceramic or single crystal materials in tension. In a prior art mechanically induced stress application, the stress is calculated to be that which is optimal for insuring piezoelectric material life in the operating conditions of the application. These operating conditions can include varying environmental temperatures and pressures.
It is also known to include an electrically controllable stress element in combination with a piezoelectric single crystal material. This element can be either a piezoelectric (voltage driven) element or a magnetostrictive effect (MS) element. These hybrid magnetostrictive-piezoelectric transducer systems are known to work effectively in a linear region.
In both mechanical and electrical stress generation means, applications avoid utilizing stress near the phase transition stress level of relaxor ferroelectric single crystals.